1. Field of the Invention
This invention relates to video display apparatuses, and more particularly, to a color subcarrier generator for use with a microcomputer system in which color data is converted into a standard color video signal for display on a visual display apparatus.
2. Description of the Prior Art
In the NTSC television system, luminance and chrominance signals are interleaved together on a color subcarrier signal included in the composite video signal. The color subcarrier signal is located at the 455th odd harmonic of one-half the horizontal sweep rate, and has an approximate frequency of 3.58 MHz. The following equation expresses the relationship between the frequency of the color subcarrier f.sub.sc and the frequency f.sub.H of the horizontal sweep: ##EQU1##
In a microcomputer system in which color data are displayed on a visual display apparatus such as a color cathode ray tube, if the total number of picture elements for one line is N, the following equation expresses the relationship between the frequency f.sub.dot of a dot clock signal used for clocking the dots comprising one line of a character and the frequency f.sub.H of the horizontal sync signal: EQU f.sub.dot =N.times.f.sub.H ( 2)
In prior art systems, separate oscillators have been used to generate the frequencies f.sub.dot and f.sub.sc. It has been difficult to satisfy the relations expressed in equations 1 and 2 by using separate oscillators, however, since the frequencies f.sub.dot and f.sub.sc cannot be easily synchronized.
The use of separate, unsynchronized oscillators to generate f.sub.dot and f.sub.sc poses significant problems. The color on the display apparatus can flicker. A moire pattern can also be formed around a displayed figure.
One proposal uses a single oscillator instead of two in order to overcome the problems with two oscillators. The output signal from the single oscillator is divided to derive both the clock frequency and the color subcarrier frequency. In other words, the signal from the oscillator is divided by m to derive the color subcarrier frequency f.sub.sc, and is divided by n to derive the clock frequency f.sub.dot. If the frequency of the oscillator is f.sub.0, the following equation represents the relationship between the frequency of the oscillator and the frequency of the color subcarrier generator: ##EQU2## The following equation expresses the relationship between the frequency of the oscillator and the frequency of the clock signal: ##EQU3## Substituting in equations 3 and 4, the following relationship is derived: ##EQU4## The following expression for m is derived from equations 1 to 5: ##EQU5## In equation 6, it is to be appreciated that the numbers n and N must be multiples of 5, 7 or 13 so that m is an integer. If N, the number of picture elements on one line, is an arbitrary number, for example, 1024 or 1000, the number n must be an integral multiple of 455 (5.times.7.times.13) for m to be an integer. Accordingly, the frequency f.sub.0 of the oscillator must be very high, in the range of 7 to 8 GHz, if N is an arbitrarily large number. However, such a high frequency oscillator is undesirable because of unnecessary radiation, the difficulty of making the necessary frequehcy divisions, and the like. One solution to this problem has been to restrict the total number of picture elements to an integral multiple of 455 (or a subcombination thereof, for example, 65 or 91) so that the number n is an arbitrary value and the frequency f.sub.0 is not quite so high.